Levels of cytosol estrogen receptors (R) and nuclear estrogen-receptor complexes (ER) in rat mammary tumors induced by 7, 12-dimethylbenz-(a)anthracene were estimated during the host's estrous cycle. Cytosol R was measured by the method of charcoal absorption employing the principle of Scatchard. Nuclear ER was determined by a [3H]estradiol exchange method. Levels of cytosol R were low at proestrus and estrus and increased significantly during metestrus-diestrus. Levels of nuclear ER were slightly elevated at diestrus-2, substantially higher at proestrus, and relatively low during other stages of the cycle. The high levels of nuclear ER during diestrus-2 and proestrus coincided with the reported rising levels of circulating estrogen in the host animals. Furthermore, rates of [3H]leucine incorporation by the tumor tissue correlated significantly with levels of nuclear ER in the corresponding tumors. The low level of cytosol R observed at proestrus could be explained by a depletion of R following the high level of serum estrogen prior to ovulation in the animal. However, the low level of cytosol R noted at estrus could not be accounted for by this estrogen-induced depletion of R, since the concomitant increase in nuclear ER was absent. Results of this study indicate that, during the normal estrous cycle in rats, levels of cytosol R and nuclear ER in mammary tumors undergo cyclic fluctuations. The change in levels of nuclear ER correlates with the tumor’s protein-synthesizing capacity as assessed by the rate of [3H]leucine incorporation. The low level of cytosol R observed at estrus represents a delay in the replenishment of R in the tissue after the exposure to a saturating amount of estrogen at proestrus.
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